The invention concerns a superconductor element comprising Nb3Sn, in particular, a multi-filament wire, which contains at least one superconducting filament which is obtained after drawing via a solid diffusion reaction from an initial filament structure which comprises a longitudinal hollow tube of niobium or a niobium alloy, in particular, NbTa or NbTi with an inner surface and an outer surface, wherein the tube is embedded in a thermally and electrically highly conductive metal matrix, in particular, of copper, the tube being filled with a material containing tin.
Superconductor elements comprising Nb3Sn are generally produced through a PIT (power-in-tube) process. Since the Nb3Sn alloy is extremely brittle, it cannot be subjected to a drawing process. If it is nevertheless desired to produce a superconductor element from this material having excellent superconducting properties, a tube containing niobium is initially filled with tin powder and is drawn followed by formation of Nb3Sn via a diffusion process. The tin contained in the powder thereby diffuses into the niobium tube.
Superconductor elements produced through PIT processes are often used to produce superconducting coils. For large current densities and large magnetic fields, coils of this type are subjected to large Lorentz forces which must be accommodated by the conductor without being damaged. This is a substantial problem, in particular, with extremely thin filaments. To ensure stable operation of the superconductor elements, the filaments should be as thin as possible. The filigree design of the filaments creates problems with respect to their stability. Moreover, during drawing, the cross-sectional shape of the inner surface of the niobium tube does not change in the same manner as its outer surface. In consequence thereof, the wall thickness of the niobium tube can vary at various locations after the drawing process and the tin which diffuses through the niobium tube reaches the copper matrix at different speeds and may even diffuse into the copper matrix and reduce the conductivity of the copper, which impairs the load capability of the filament. For this reason, the amount of tin is usually minimized to prevent diffusion of the tin into the copper matrix. However, not all of the available niobium material can thereby react to form Nb3Sn and correspondingly smaller amounts of superconducting material are thereby produced. Alternatively, one must provide a correspondingly larger amount of initial material. This increases the diameter of the filaments.
The documents “Filamentary A15 Superconductors” Masaki Suenaga; Alan F. Clark; PLENUM PRESS NEW YORK AND LONDON, 1980; pages 17-22 and the data sheet “Low-temperature superconductors” of the company European Advanced Superconductors GmbH & Co. KG (see website bruker.de/eas) disclose superconductor elements havnig niobium filaments embedded in a bronze matrix, wherein the bronze matrix is surrounded by a highly conductive copper layer. To avoid tin impurities in the copper, a tantalum foil is provided as a diffusion barrier and is disposed between the bottom matrix and the copper.
It is the object of the present invention to propose a superconductor element produced by a PIT process with improved stability and high current-carrying capacity. A further object of the invention consists in proposing a method for producing a superconductor element of this type.